This invention relates to a ballast, and more particularly, to an improved method and apparatus for controlling a ballast to drive a lamp. The invention has particular application in the control of high intensity discharge (HID) lamps.
High intensity discharge lamps typically operate in an alternating current (AC) mode when in steady state. More specifically, these lighting devices are typically driven with a square wave that alternates between a plus and minus voltage, so that in a steady state, current is not flowing in one direction for more than a few milliseconds. An exemplary frequency at which such a device might operate would be 60 Hz. Due to the physics and design of such devices, if current flow is in one direction for too long, damage to the device will occur.
U.S. Pat. No. 5,917,290 (xe2x80x9cthe ""290 patentxe2x80x9d) describes a technique for driving an HID lamp. FIG. 1 hereof shows a circuit similar to that of the ""290 patent. The circuit of FIG. 1 includes switches 103-108, which may conveniently be implemented as transistors in the preferred embodiment. A controller 165 controls the on and off states of the switches 103-108, preferably by setting appropriate gate voltages. For purposes of clarity, we do not show in FIG. 1 all of the connections from controller 165 to the switches 103-108.
In the arrangement of FIG. 1, two relatively high frequency (e.g., 200 kilohertz) square waves are produced by controller 165. One of those high frequency square waves is utilized to drive the gates of switches 103 and 104, at different times, depending upon whether the alternating input voltage VAC is positive or negative. The other high frequency signal from controller 165 is used to drive the gates of transistors 107-108, depending upon the polarity of the alternating voltage across lamp 115. Switches 105-106 are operated at the frequency of VAC, the same frequency as the voltage across lamp 115. This frequency is typically low, under 100 Hz.
It can be appreciated that the frequency of the signal driving the HID lamp 115 is at the same frequency as the input voltage VAC, which corresponds to the frequency at which switches 105 and 106 are operated. While the foregoing is acceptable in many applications, it is unacceptable if the line frequency of the input voltage VAC is too low. The situation may arise, for example, in Europe, where the line frequency is only 50 hertz, slightly below the minimum frequency at which HID lamp 115 needs to be driven in order to provide satisfactory operation. As a result, a circuit such as that shown in FIG. 1 may cause flicker in the lamp when operated in the European market. Other situations may also make it desirable to operate the lamp using an AC signal at a frequency different from that of the input voltage.
In view of the above, there exists a need in the art for a system which is both simple in design and which may drive an HID lamp or other device at a frequency other than the line frequency.
The above and other problems of the prior art are overcome in accordance with the teachings of the present invention, which relates to a technique of generating a voltage to drive the HID lamp, which is at a different frequency than that of the input voltage. For purposes of explanation, we refer herein to pulse width modulation (PWM) switches, or high frequency switches. These switches, implemented in the preferred embodiment as transistors, are typically driven with a square wave that is at a much higher frequency (e.g. 200 KHz) than the frequency of the AC voltage across the lamp.
Also for purposes of explanation, we refer herein to a boost and a buck. These terms are known in the art. The boost typically refers to conversion of power from low voltage to high voltage, and the buck refers to conversion of voltage from high voltage to low voltage. In ballasts such as that described herein, the input voltage is usually first put through a boost phase, and is then xe2x80x9cbuckedxe2x80x9d prior to being applied to the lamp. Thus, the input voltage is first stepped up, and then stepped down.
In accordance with the invention, the control circuitry that drives switches such as those of FIG. 1 is configured in a manner such that the arrangement has an in-phase and out-of-phase mode. During the in-phase mode, two different switches operate at high frequency but in synchronization to produce PWM signals. One of the two operates to perform the buck function, and the other operates to perform the boost function. In the out-of-phase mode, a single switch operates in the high frequency PWM mode, and serves to implement the boost and the buck functions.
For each positive portion of the input voltage, there can be an in-phase and out-of-phase lamp voltage, and thus, the lamp voltage may be at twice the frequency as the input voltage. In general, the lamp voltage may operate at any frequency that may be different from the input voltage to the circuit.
The invention generally includes a control function that periodically changes between using a single PWM switch to implement the boost and buck functions, and using two PWM switches in synchronization, one for the boost function and one for the buck function.
In a system for driving a discharge lamp a power source and a controller each produce a separate periodic signal, and wherein a period associated with the controller""s signal divides equally into a period associated with the power source""s signal.